This invention relates generally to a power supply for electronic devices, and more particularly, to an electronic timepiece with a control circuit having a power supply including a power generating device and boosting circuits.
A conventional electronic device with a power generating device is described in Japanese Laid-open Patent Publication No. 60-203887. This device steps up the voltage from a secondary power source by activating a high capacitance condenser (hereinafter "a capacitor") which stores generated electric energy, and uses the stored energy in the form of a boosting voltage to act as a power source. This arrangement assures longer operation time of the electronic device. If the capacitor voltage is designated as a primary voltage and the output voltage resulting from the boosting circuit is designated as a secondary voltage, the secondary voltage level is used as the control signal of the circuit.
However, conventional electronic devices having power generating devices of the prior art have technical problem due to the switching transistors employed in the boosting circuit. When MOSFET type switching transistors are employed, current drawn by them is expressed by the following equations:
In the saturation region, EQU I.sub.d =.beta.(V.sub.GS -V.sub.TH).sup.2 /2 Equation 1
In the non-saturation region, EQU I.sub.d =.beta.{(V.sub.GS -V.sub.TH)V.sub.DS -2V.sub.DS.sup.2 }/2Equation 2
When the capacitor voltage reaches a certain voltage level region or when the boosting voltage is low, V.sub.GS is smaller than V.sub.TH in the above equations. Therefore, the transistors fail to conduct and the voltage boosting operation is shut down. A shut down of the voltage boosting operation is detrimental to the electronic device, especially in the case when the electronic device employs a capacitor charged by the boosting voltage as the main source of power. When this state occurs, the electronic device entirely shuts down.
Accordingly, it is desirable to provide an electronic device with a reliable operation of the boosting circuit to prevent the electronic device from shutting down when the capacitor voltage reaches a certain voltage level region or when the boosting voltage is low.